The present invention relates to an analog switching circuit provided with an input clamping circuit.
Referring to FIG. 1, a conventional analog switching circuit with two input terminals is shown. In FIG. 1, reference characters Q.sub.1A, Q.sub.1B, Q.sub.2A, Q.sub.2B, Q.sub.3A, Q.sub.3B, Q.sub.4A and Q.sub.4B denote transistors; I.sub.A, I.sub.B, constant current sources; R.sub.1A, R.sub.1B, R.sub.2A, R.sub.2B and R.sub.L, resistors; A and B, input terminals; OUT, the output terminal; V.sub.1, a bias voltage; V.sub.A, V.sub.B, control signals having reversed phase with respect to each other; and V.sub.cc, the power supply voltage.
When the control signal V.sub.A is at a high level "H", the transistor Q.sub.4A assumes the ON state. Then the transistor Q.sub.2A becomes OFF because the base of the transistor Q.sub.2A is grounded, and the transistor Q.sub.1A functions as an emitter follower and a base bias voltage is applied to the transistor Q.sub.3A. Therefore, the transistor Q.sub.3A also functions as an emitter follower and the input signal A is output from the output terminal OUT through the transistors Q.sub.1A and Q.sub.3A. The DC voltage of the output is cancelled by the voltage between the base and emitter of the transistors Q.sub.1A and Q.sub.3A, and it becomes approximately equal to the input DC bias voltage V.sub.1.
On the other hand, on the side of the input signal B, the control signal V.sub.B is at a low level "L" and the transistor Q.sub.4B is in the OFF state. The base voltage of the transistor Q.sub.2B is increased through the resistor R.sub.2B connected to the power supply voltage V.sub.cc, so that the transistor Q.sub.2B turns ON, and in turn the transistor Q.sub.1B is made OFF because of the increment to the emitter voltage of the transistor Q.sub.1B. Therefore, the input signal B is not transmitted to the output terminal OUT, and the analog switching circuit is in an OFF state.
There is a problem in that an inverse breakdown current flows between the base and the emitter of the transistor Q.sub.1B when the voltage between the bases of the transistors Q.sub.1B and Q.sub.2B exceeds the sum of the voltage between the base and emitter of the transistor Q.sub.2B and the Zener voltage between the emitter and base of the transistor Q.sub.1B. This causes lowering of the h.sub.FE of the transistor Q.sub.1B, and in the case where the control signal V.sub.B is a high level "H" and the input signal B is transmitted to the output terminal OUT, the input bias current undesirably increases, thereby introducing deterioration of the circuit characteristics. This is especially notable when the power supply voltage V.sub.cc is high.
It has been considered to resolve the above problem by clamping the control voltage to be applied to the bases of the transistors Q.sub.2A and Q.sub.2B by means of a Zener diode. A portion of a conventional analog switch having such a Zener diode is shown in FIG. 2.
Referring to FIG. 2, Q.sub.1 denotes an input transistor for receiving an input A at its base; Q.sub.2, a switching transistor having the same polarity as the input transistor Q.sub.1, the emitter of the switching transistor Q.sub.2 being connected in common with the emitter of the input transistor Q.sub.1 ; Q.sub.4, a control transistor having its emitter connected to ground and its collector connected to the base of the switching transistor Q.sub.2 for controlling the ON and OFF states of the switching transistor Q.sub.2 in accordance with the control signal V.sub.A ; Q.sub.3, an output transistor having its collector connected to ground and its base connected to the common emitter of the transistors Q.sub.1 and Q.sub.2 for operating the emitter follower thereof; I, a constant current source connected between the common emitter of the transistors Q.sub.1 and Q.sub.2 and ground; V.sub.1, a bias voltage applied to the transistor Q.sub.1 ; R.sub.1, R.sub.2 and R.sub.L, resistors; and ZD, a Zener diode for providing a base bias equal to a Zener voltage to the switching transistor Q.sub.2 when the control transistor Q.sub.4 is in the OFF state.
Where the control transistor Q.sub.4 is in the OFF state, namely in a switch-OFF mode where the switching transistor Q.sub.2 is in the ON state and the input signal A is not transmitted, the base voltage of the switching transistor Q.sub.2 becomes equal to the Zener voltage V.sub.z because the switching transistor Q.sub.2 is biased by the power supply voltage V.sub.cc through the resistor R.sub.2 and the Zener diode ZD. At this time, if the input signal A exceeds the Zener voltage V.sub.z, the input transistor Q.sub.1 switches from the OFF state to the ON state, and then the switching transistor Q.sub.2 switches to the OFF state, thereby undesirably transmitting the input signal A to the output terminal. Namely, the input signal A is undesirably transmitted even though the analog switching circuit is in the switch-OFF mode. If the power supply voltage V.sub.cc is high, the base bias voltage V.sub.1 of the input transistor Q.sub.1 is also increased in proportion to the power supply voltage, and therefore the dynamic range of the input signal A is undesirably limited to a narrow range V.sub.z -V.sub.1 and a small amplitude, which constitutes a defect of the conventional analog switch.